1. Field of the Invention
This invention relates to improvements in speed control systems for direct current motor driving circuits.
2. Prior Art
Conventionally it is known that motor speeds are controlled by detecting the induced voltage of a generator which is directly coupled to the motor rotating shaft or by indirectly detecting the induced voltage of a motor by use of a bridge servomechanism in which one of the unknown resistors of the Wheatstone bridge is replaced with the motor. With the former system, however, the generator makes the motor load heavy, which causes the power consumption and system size to increase. On the other hand, the latter has the following disadvantages: The power dissipated at each resistor of the bridge cannot be neglected. Also since each resistance varies with temperature changes, a thermistor and such are necessary to compensate for this variation. Furthermore, because of its aging effect, the motor is subject to change in commutator-and-brush contact resistance and considerable variation in rotating speed caused by the aging drift.
FIG. 1 is a block diagram of an embodiment of the conventional motor driving circuit, in which reference numeral 101 is a wave shaping circuit comprising an integrating circuit for smoothing the waveform; 102 is a driving circuit; 103 is a motor; 104 is a generator directly coupled to the motor shaft; and 105 is a detecting circuit for detecting the induced voltage of the generator. The variation in motor rotating speed is detected as the variation in the induced voltage of the generator and is made smooth through the wave shaping circuit so as to vary the bias current of the driving circuit and provide a fixed speed. The disadvantage of the system, as mentioned above, is that since the generator is directly coupled to the motor shaft, the motor load and power consumption are increased. In addition, the use of an integrating circuit as a wave shaping circuit not only increases the time constant in the control system but also degrades the response.